EDFAs (Erbium-Doped Fiber Amplifiers) using an EDF (Erbium-Doped Fiber) are widely used as an optical amplification technique for WDM (Wavelength Division Multiplexing) transmission systems. An EDFA can amplify a band of 1530-1565 nm (C-Band) or a band of 1570-1605 nm (L-Band). An EDFA is characterized by a wide band. This characteristic is exploited and wavelengths are arranged in these bands at intervals of 0.4 nanometers. By doing so, WDM transmission systems which can transmit signals with eighty or more different wavelengths are put to practical use.
In an optical transmission system an optical amplifier is used for compensating for transmission loss. A signal light transmitted from a transmitter propagates along an optical fiber which is a transmission line, and is received by a receiver. A postamplifier placed at an output stage of a transmitter, a preamplifier placed at an input stage of a receiver, an in-line amplifier used in the case of multistage relay and the like are known as forms of optical amplifier placement.
For example, a variable gain optical amplifier is used as a WDM optical amplifier. With a variable gain optical amplifier control is exercised so that the gain of an EDF will be constant. By doing so, the gain-wavelength characteristic of the EDF does not change. Gain control is exercised by changing loss in a VOA (Variable Optical Amplifier) on the output side. With this variable gain optical amplifier output increases with an increase in the power of input to the EDF. Accordingly, a very strong excitation light is necessary.
Usually the gain of an EDF is not flat with respect to wavelengths. Therefore, a GEQ (Gain Equalizer) is used for equalizing the gain of an EDF. For example, a GEQ has a loss-wavelength characteristic which makes the output gain of an EDF flat with respect to wavelengths. GEQs using a dielectric multilayer film are widely used.
In the past, a long-cycle fiber grating by which the tilt of a transmission loss-wavelength waveform of an EDFA changes according to the tilt of a gain-wavelength waveform and by which the tilt of the above gain-wavelength waveform is arbitrarily controlled was proposed (see, for example, Japanese Laid-open Patent Publication No. 2004-103682).
Furthermore, an optical component, an optical amplifier module, and an optical transmission system which can compensate for a gain tilt to a higher degree was proposed (see, for example, Japanese Laid-open Patent Publication No. 2004-101935).
In addition, an optical amplifier control apparatus which has a simple structure and which speedily controls transient fluctuations in signal levels caused by SHB (Spectral Hole Burning) or SRS (Stimulated Raman Scattering) without deteriorating a noise characteristic was proposed. This optical amplifier control apparatus makes it possible to realize a multistage optical amplifier, and therefore realizes a long-distance transmission system including an optical add-drop multiplexer (see, for example, Japanese Laid-open Patent Publication No. 2006-295113).
However, a gain tilt of an EDF occurs according to set gain. A loss-wavelength characteristic of a GEQ is fixed. Accordingly, gain of the EDF is not flat with respect to wavelengths, depending on gain of the EDF set.